The burden of the demands for the improved performance of computers and other electronic devices falls on the lithographic processes used to fabricate integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer brings about advances in performance. The enhanced resolution required to achieve finer line-widths is accomplished by decreasing the wavelength of the illumination source. As a result, the energies used in lithographic patterning are moving deeper into the UV region. In particular, projection optical photolithography systems and <200 nm excimer laser systems that utilize vacuum ultraviolet (“VUV”) wavelengths of light at and below 200 nm provide desirable benefits in terms of achieving smaller feature dimensions. Consequently, optical components capable of reliable performance at short optical microlithography wavelengths are required.
Few materials are known to both have a high transmittance at wavelengths below 200 nm (for example, at 193 nm and 157 nm) and also not deteriorate under intense laser radiation exposure. Fluoride crystals such as those of magnesium fluoride, calcium fluoride and barium fluoride are potential materials for use at wavelengths <200 nm.
The commercial use and adoption of 193 nm and below vacuum ultraviolet wavelengths has been hindered by the transmission nature of such deep ultraviolet wavelengths through optical materials. The slow progression in the use of VUV light below 200 nm by the semiconductor industry has been also due to the lack of economically manufacturable, high quality blanks of optically transmissive materials suitable for making